Spread-F radar observations in the midlatitude ionosphere using an ionosonde-radiodirection finder

摘要

We present the results of experimental studies of the spread-F phenomenon in the midlatitude ionosphere, which were obtained using a broadband chirp HF radar of bistatic configuration on the Cyprus - Rostov-on-Don oblique sounding path. In the evening, night, and early morning hours of the winter and spring months, in a quiet geomagnetic environment, scattered signals were detected at frequencies exceeding the maximum observable frequency (MOF) of the directsignal 1F mode with the following parameters: delays of 6-7.5 to 8.5-11 ms, frequencies of 10-14 to 13-19 MHz, vertical angles of arrival of 20 to 45, azimuthal angles of arrival of -30 to 50, and amplitude of scattered signal by 70-80 dB less than the direct-signal amplitude. Based on modeling of HF propagation and scattering and on comparison with the experimental data, it is established that the observable anormalous signal is due to the backscattering of radio waves by small-scale irregularities with sizes about 8-10 m, which occupy an extended area of the midlatitude ionosphere (50 -55 N, 35 -48 E) at altitudes both below and above the F-layer maximum (250-450 km). Relative fluctuations of the electron number density of the scattering irregularities are estimated as δN ≈ 7.8 10~(-3), which is about a factor of 3 to 5 greater than δN for natural fluctuations of the electron number density under typical conditions in the midlatitude ionospheric F region. Possible relation between spread F and the traveling ionospheric disturbances is discussed.